Crystallization and Porosity as Clues to Distant Activity of Comets

D. Prialnik (Tel Aviv University)

Comets are often found to be active at heliocentric distances far beyond the limit of tex2html_wrap_inline11 AU, within which the activity may be explained by sublimation of water ice, induced by insolation. The exothermic transi- tion from amorphous to crystalline ice has long been recognized as a suitable mechanism for explaining such distant bursts of activity.

Indeed, cometary ice, formed at low temperatures and pressures, should be amorphous and trap large amounts of gas (CO, CO tex2html_wrap_inline13 , etc.), which is subsequently released during crystallization. Recent observations of comets indicate a highly porous structure, which is permeable to gas fluxes. The free gases present in the interior of a comet, due to subli- mation (in response to internal heating) and release of trapped gas, gives rise to internal pressures, which may surpass the tensile strength of the already fragile grainy configuration. This may result in cracking of the ice matrix and outbursts of gas and dust.

It is shown that the expected effect of crystallization in comets can be assessed based on simple analytical arguments, involving the time scales of competing processes. When crystallization of amorphous ice is inclu- ded in detailed numerical models of the evolution of comet nuclei, it is found that the process is not continuous, but progresses in spurts. Their onset, duration and extent in depth are largely determined by the structure, composition and thermal properties of the nucleus, and, of course, by the comet's orbit. The results of several calculations that support and develop this hypothesis are shown (P/Halley, 2060 Chiron, Hale-Bopp).